Stock-based Awards - Stock Options (Details) (Options to purchase common stock, USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	6 Months Ended
	Dec. 31, 2013	Jun. 30, 2014
Options to purchase common stock
Number of Options
Outstanding (in shares)		18,363,144
Granted (in shares)		10,545,647
Exercised (in shares)		(1,741,058)
Canceled/forfeited (in shares)		(502,943)
Outstanding (in shares)	18,363,144	26,664,790
Vested and expected to vest (in shares)		25,236,587
Exercisable (in shares)		18,884,427
Weighted-Average Exercise Price
Outstanding (in dollars per share)		$ 4.89
Granted (in dollars per share)		$ 15.86
Exercised (in dollars per share)		$ 1.01
Canceled/forfeited (in dollars per share)		$ 9.36
Outstanding (in dollars per share)	$ 4.89	$ 9.40
Vested and expected to vest (in dollars per share)		$ 9.32
Exercisable (in dollars per share)		$ 8.72
Additional disclosures
Weighted Average Remaining Contractual Life, Outstanding	7 years 2 months 1 day	7 years 11 months 23 days
Weighted Average Remaining Contractual Life, Vested and expected to vest		7 years 10 months 28 days
Weighted Average Remaining Contractual Life, Exercisable		7 years 4 months 21 days
Remaining compensation expense for unvested awards		$ 73.3
Recognition period		3 years 1 month 6 days